It is well known to provide in such an alternator circuit a warning lamp which is arranged on the fascia, i.e., the dashboard of the vehicle for indicating in particular a fault in the auto-excitation of the alternator. The lamp is conventionally connected between the voltage V.sub.D+ which is delivered by the auxiliary rectifier bridge (three diodes) of the alternator, and the voltage V.sub.B+ at the positive terminal of the battery. It is extinguished only when these voltages are approximately equal.
There is at the present time an increasing tendency to use for motor vehicles a double electrical network comprising two alternators associated with two batteries. Such a double voltage network has a number of advantages. Firstly, the various items of electrical equipment on the vehicle demand a large consumption of power, and by doubling the supply voltage available--the double voltage network is capable of delivering 28 volts, for example, between two of its terminals instead of the usual 14 volts--the double voltage circuit enables the current which is used to be essentially divided between the two alternators and batteries, which enables electrical conductors to be used which are of smaller cross section and therefore cheaper.
In addition, in connection with the supply of electric motors of the kind which require to be reversed, the associated switching means are simplified by virtue of the availability of two voltages, namely .+-.14 volts.
Finally, where the vehicle has headlamps equipped with discharge lamps, the high tension supply circuit for these lamps can be derived from a d.c. voltage which is twice as large as in conventional systems. Accordingly, certain components, and in particular the step-up voltage transformer which is usually provided in such circuits, may be selected so as to be of reduced size and cost.
An immediate approach, as far as fault indication in such a double voltage network is concerned, is to provide two warning lamps, associated respectively with the two alternator circuits. However, it is undesirable difficult or both to locate two lamps on the dashboard, either for reasons of selling price or because there is not room for them.